_External epiblast._--Division into separate layers. Placoid scales. Formation of their enamel. _Lateral line._--Previous investigations. Distinctness of lateral line and lateral nerve. Lateral nerve a branch of vagus. Lateral line a thickening of epiblast. Its greater width behind. Its conversion into a canal by its cells assuming a tubular arrangement. The formation of its segmental apertures. Mucous canals of the head. Their nerve-supply. Reasons for dissenting from Semper's and Götte's view of lateral nerve. _Muscle-plates._--Their growth. Conversion of both layers into muscles. Division into dorso-lateral and ventro-lateral sections. Derivation of limb-muscles from muscle-plates. _Vertebral column and notochord._--Previous investigations. Formation of arches. Formation of cartilaginous sheath of notochord and membrana elastica externa. Differentiation of neural arches. Differentiation of hæmal arches. Segmentation of cartilaginous sheath of notochord. Vertebral and intervertebral regions. Notochord. CHAPTER VIII. DEVELOPMENT OF THE SPINAL NERVES AND OF THE SYMPATHETIC NERVOUS SYSTEM, pp. 378-396. _The spinal nerves._--Formation of posterior roots. Later formation of anterior roots. Development of commissure uniting posterior roots. Subsequent development of posterior roots. Their change in position. Development of ganglion. Further changes in anterior roots. Junction of anterior and posterior roots. Summary. _General considerations._--Origin of nerves. Hypothesis explaining peripheral growth. Hensen's views. Later investigations. Götte. Calberla. Relations between Annelidan and Vertebrate nervous systems. Spinal canal. Dr Dohrn's views. Their difficulties. Hypothesis of dorsal coalescence of lateral nerve cords. _Sympathetic nervous system._--Development of sympathetic ganglia on branches of spinal nerves. Formation of sympathetic commissure. CHAPTER IX. DEVELOPMENT OF THE ORGANS IN THE HEAD, pp. 397-445.